Metal Oxide TFTs – Closer to Realization


In recent years we have seen flat screen TVs growing bigger and bigger and their capabilities continuously improved. Full high definition (HD) screen resolutions of (1920x1080 pixels) are now standard with many panel makers showcasing or announcing larger ultra-definition resolution screens. At the same time refresh rates are increasing, providing a better appearance of moving images and 3D TV.

Consumers today can choose from a large variety of huge screens providing extremely crisp and detailed images at very high refresh rates. Most recently, large OLED TVs have started to reach the market. However, fabricating these high-performance displays represent a big challenge to the panel makers since they need to figure out a way to drive all these pixels integrated on very large areas with sufficient speed and low power consumption.

The pixels are driven by an array consisting of millions of thin-film transistors (TFTs) called a backplane. Amorphous silicon thin film transistor (a-Si TFT) backplanes have been the workhorse of the display industry for many years. However, a-Si has intrinsic limitations that make it difficult to provide the switching characteristics necessary to drive high resolution, high performance displays. A-Si transistors are just too slow to do the job: electrons and holes move too slowly with the material, a property we call mobility.

A couple of technologies capable to overcome the mobility obstacles of a-Si TFTs are on the horizon. Among those, metal oxide TFT technology seems to be best suited to win the race for several reasons (for a comprehensive overview I refer to my colleague Kerry Cunningham who elaborated extensively on metal oxide technology in a recent blog post). The technology can be quite easily adopted by existing TFT fabs, since the metal oxide is usually laid down using physical vapor deposition (PVD), which is already well known. The materials under consideration can be deposited without any size constraint and provide sufficiently high mobility to make backplanes suitable for high resolution, high refresh rate flat screen displays.

When I attended the International Meeting on Information Display 2012 (IMID) a few weeks ago in Daegu, South Korea, a lot of progress was reported on metal oxide TFTs. The favorite material for the active layer is indium gallium zinc oxide (IGZO), which provides mobilities 10-20 times higher than that of a-Si.

Applied Materials' Rotary Cathode Array

IGZO is usually deposited by a PVD technique called magnetron sputtering, where a combination of magnetic and electric fields are used to remove material from the cathode and deposit it on the backplane. There are two types of cathode: planar and rotary. Rotary cathodes, as the name implies, rotate during deposition to enable more of the expensive deposition material to be consumed before the cathode must be replaced, which lowers costs. The availability of large rotary cathodes made from the material of choice, IGZO, makes Applied Materials’ deposition technology the ideal choice for developing processes for metal oxide layers.

Still there are some challenges to overcome before metal oxide TFTs are mature enough for production purposes, particularly when it comes to making OLED TV backplanes. Key areas for improvement include “mura effects,” where different parts of the display appear brighter or darker, and device instability. If either effect is strong enough to be visible it will disturb the consumer’s viewing experience.

Mura effects can be readily addressed by using Applied's PVD technology, which has the unique ability to optimize the distribution of the material sputtered from the cathodes onto the substrate by changing the direction of the sputter source using a moving magnet.

The key to improving the stability of the TFT device is to protect the metal oxide active layer from hydrogen contamination during the patterning process that defines the shape of each transistor. The active layer is protected using a process called passivation, where a thin layer of an insulating material such as aluminum oxide (Al2O3) is deposited. The Al2O3 layer is often built up one monolayer at a time by atomic layer deposition (ALD), but this process is slow and therefore costly. As a result, standard PVD processes have gained interest recently due to much higher productivity at similar performance.

Typical metal oxide thin-film transistor structure

As you see, the field of metal oxide TFTs is very dynamic which makes it exciting to work on materials, processes and architectures for this next generation TFT backplane technology. For consumers this is enabling the next generation of exciting displays.